1. Field of the Invention
This invention relates to induction meters having an electromagnetic assembly including a voltage magnetic section and a current magnetic section and more particularly to the voltage magnetic section of such a meter having an integral, self-aligned light load adjustment assembly.
2. Description of the Prior Art
Induction meters such as watthour meters are almost universally used for billing and load monitoring of AC electric energy transmitted by an electric energy supplier to residential and commercial electricity users. Voltage and current magnetic sections which comprise the meter's electromagnetic assembly produce magnetic fluxes responsive to line voltage and line current. The magnetic fluxes are directed into an electroconductive disk which rotates in response to the consumption of AC electric energy. In the manufacture and design of watthour meter electromagnetic assemblies, it is known to provide adjustments on the voltage magnetic section so that the voltage and current fluxes rotate the disk at a calibrated rate or speed having a predetermined ratio to the consumption of electric energy as measured in kilowatt hours. The ratio of the disk speed to the measure electric energy is often referred to as the watthour constant of the meter.
Correction and adjustment of the power factor or voltage flux phase lag and light load characteristics are two common calibrating adjustments of induction watthour meters. The calibrations are used to provide operation in accordance with the well-known basic theory of operation of induction watthour meters which includes the principle that the magnetic flux from the voltage section, being responsive to the line voltage, must lag the magnetic flux from the current section, being responsive to the line current, by ninety electrical degrees when the line voltage and current are in phase at unity power factor. In modern watthour meters, the voltage and current fluxes are directed in this quadrature phase relationship in opposing directions into an airgap and into opposite sides of the meter disk which is located in the airgap. The fluxes produce eddy currents in localized areas of the disk. In accordance with the aforementioned basic theory, interaction of the eddy currents with the voltage and current magnetic fluxes develops a torque on the meter disk in proportion to the measured electrical energy.
The voltage magnetic section typically has an E-shaped magnetic core carrying a coil having a large number of winding turns of a small conductor on the center leg thereof. The voltage coil has substantial inductance so that the voltage airgap flux produced by the coil will substantially lag the line voltage, but not by the required ninety degrees without further adjustment. To provide the additional flux lagging adjustment, it is known to provide a loop of conductive material around the voltage flux so that circulating current is induced therein to establish a further voltage flux component that is vectorially added to the voltage coil main flux component to produce a corrected working voltage flux in the airgap. The resultant airgap voltage flux has substantially the aforementioned ninety degrees or quadrature lagging relationship with the line voltage and current flux at unity power factor.
Light load adjustment differs from the phase lag adjustment so that at low values of line current an asymmetrical component of voltage flux will add torque to the meter disk. A part of the light load adjustment compensates for the slight frictional drag on the shaft which carries the meter disk and another part compensates for the non-linear and lower permeability characteristics of the laminated magnetic core materials at low magnetic induction causing the line current related flux to be disproportionately low. Compensation and calibration of the light load adjustment in the voltage magnetic section of some electromagnetic assemblies is related to the regulation of the disk torque produced by the magnetic drive effects of an asymmetrical voltage flux path established between the main center voltage pole face and a pair of auxiliary voltage pole faces. A secondary voltage flux path is divided and extends between the disk and the pair of auxiliary voltage pole faces formed at the free ends of the opposite legs of the E-shaped voltage core on opposite sides of the main voltage pole face. The secondary flux path produces a large magnetic flux shunting path to carry the leakage or the non-driving component of the voltage flux. A light load adjuster is described in U.S. Pat. No. 2,947,942 which includes two magnetic members movable over the auxiliary pole faces. The magnetic members produce dissymmetry in the disk driving flux by producing effects in the driving flux path in a predetermined adding or subtracting relationship (depending on the position) to the main driving torque on the disk. The magnetic members are movably mounted on a brass bracket fastened to the meter electromagnetic assembly. The bracket also carries a threaded adjusting member to move the magnetic members.
In U.S. Pat. No. 3,493,862, a light load adjuster is described including soft magnetic members carried by a nonmagnetic bracket. The magnetic members are pivotally mounted on the brackets so that the ends of the members extend over the faces of the voltage flux arms to vary light load calibrations. U.S. Pat. No. 4,213,090, is for a voltage magnetic section of a watthour meter having a voltage flux lagging loop formed integrally with a platform part directly attached to the voltage magnetic core. Light load adjustment members are movably mounted on the platform which also carries a manual operator for shifting the light load members to a predetermined position for proper operation in the meter.